Those defections swelled the number of lawmakers likely to back a motion against Ms Rousseff when the lower house of congress votes on Sunday on whether an impeachment trial should be launched.

In Wednesday's interview she branded the motion "a fraud." She would not rule out appealing to the Supreme Court to dismiss it if the congress votes for it to move forward.

Polls published in the Brazilian media indicate opposition parties are closing in on securing the 342 votes needed to approve the impeachment motion and send it to the Senate for a further vote.

Leading newspaper O Estado de Sao Paulo calculated that the number of lawmakers who have now decided to support impeachment has risen to 302 - but dozens have yet to state a position.

Analysts say the desertion on Tuesday of two of Ms Rousseff's key allies, the PP and PRB parties, which have 69 lawmakers between them, could prompt a stampede.

"If all the medium-sized parties abandon her, Rousseff will have no way to survive impeachment," said political scientist David Fleischer of Brasilia University.

On Wednesday one of Ms Rousseff's last remaining coalition allies, the PSD party, also positioned itself against Ms Rousseff. A party source said 26 of its 36 lawmakers had decided to vote for impeachment.

Another party, the PR, was scheduled to meet on Thursday. It has 40 seats. Between them, the two parties could swing the vote against Ms Rousseff in the 513-seat Congress.

Ms Rousseff is fighting to save her presidency over charges that she illegally manipulated government accounts to mask the effects of recession during her 2014 re-election and in 2015.

On Tuesday she branded her vice-president Michel Temer a traitor after an audio recording was leaked in which he was heard practicing the speech he would make if Ms Rousseff is impeached.

"The conspirators' mask has slipped," she said.

"We are living in strange and worrying times - times of a coup, and of pretending, and betrayal of trust."

Protesters for and against Ms Rousseff have called for demonstrations this weekend in Brasilia. Security forces have put up fences to protect government buildings from possible disturbances.

Lawmakers who have yet to declare their position were facing fierce lobbying, including from Ms Rousseff's top ally and predecessor as president, Luiz Inacio Lula da Silva.

But he too faces pressure: the courts have suspended his appointment as Ms Rousseff's chief of staff over a corruption case against him, linked to a huge graft scandal at state oil company Petrobras.

The country has sunk meanwhile into its worst recession in decades.

"Deputies are thinking about their chances of being re-elected" in the next elections, scheduled for 2018, said Mr Fleischer.

Backing Ms Rousseff is politically risky since her popularity has plunged so much, he added.

If the lower house votes by two-thirds to move forward with the motion, the Senate must then hold a vote on whether to hold an impeachment trial.